In the medical field, doctors display medical images of objects (three-dimensional image data including tomographic images indicating three-dimensional information about the inside of the object) on monitors and interpret (or read) the displayed images to diagnose lesions. Examples of medical image collection devices (hereinafter referred to as “modalities”) for use in medical imaging include an ultrasonic diagnostic imaging apparatus, a magnetic resonance imaging (MRI) apparatus, and an X-ray computed tomography (CT) apparatus.
The observation of a medical image captured by an individual modality is not sufficient for correct diagnosis of the state of a lesion. Therefore, attempts have been made to correctly diagnose the state of a lesion by comparing corresponding regions of the lesion in medical images captured by a plurality of modalities or in medical images captured at different dates and times.
In order to use a plurality of kinds of medical images for diagnosis, it is important to identify (or associate) corresponding regions of a lesion or the like in the respective medical images. Since it is difficult to automatically identify such regions by using image processing due to factors such as different modalities and object deformation, an operator such as a doctor generally performs a manual operation for identifying the regions while viewing the images. The operator searches for and identifies, while viewing an image of a lesion of interest to be focused on which has been pointed out in one medical image (hereinafter referred to as the “reference image”), a corresponding region of the lesion (hereinafter referred to as the “corresponding lesion”) in another medical image (hereinafter referred to as a “target image”) on the basis of similarity in the shape of the lesion, the appearance of the surroundings of the lesion, and other properties. If an apparatus for presenting a medical image has the function of estimating the position of the corresponding lesion represented in the coordinate system of the target image from the position of the lesion of interest in the coordinate system of the reference image and presenting the estimated position, the operator is able to search for the corresponding lesion on the basis of the estimated position.
Accordingly, attempts have been made to measure the position and posture of an ultrasonic probe to determine the relationship between the coordinate system of the target image, or an ultrasonic tomographic image, and the coordinate system of the reference image, and to estimate the position of the corresponding lesion in the coordinate system of the ultrasonic tomographic image (ultrasonic coordinate system) to navigate the probe. For example, PTL 1 discloses that the distance and direction from the current ultrasonic tomographic image to the center of a target (lesion of interest) set in a reference image (cross-sectional image of a three-dimensional image obtained by an X-ray CT apparatus, an MRI apparatus, or the like) are calculated and a three-dimensional arrow image and numerical values based on the calculated distance and direction are displayed. Thus, the operator is able to visually determine the distance from the current ultrasonic tomographic image to the target, and therefore easily determine a correspondence (positional relationship) between the reference image and the ultrasonic tomographic image.
PTL 2 discloses that when a selected image tracking point (lesion of interest) is given from a previously acquired ultrasonic tomographic image (volume or slice), a square whose size and color are based on the distance and direction from the currently acquired ultrasonic tomographic image is displayed as an in-plane indicator on top of the currently acquired ultrasonic tomographic image. Therefore, when counting the number of nodules in the thyroid, the number of metastases in the liver, or the number of other items, a user can determine whether the current visualized structure is newly identified or has already been identified and counted even if the angle or location of the probe on the object changes.